Along with general-purpose applications of an electrophotographic technology, an image forming apparatus of an electrophotographic system has come into use in various image forming processes. In particular, to meet the demand for producing high color image quality, a strict requirement for performance stability is imposed, among others, on members used in the electrophotographic process, especially on an electrophotographic photoreceptor (hereinafter, sometimes referred to as “photoreceptor”) serving as a core part. Above all, in the case where the light attenuation behavior is not so much governed by the process control, a slight potential variation affects the image density, gradation and color tone and therefore, it is required that electrical and mechanical characteristics are stable throughout the life.
Various disturbance factors affect the performance of the photoreceptor and with respect to so-called light-induced fatigue characteristics such that the chargeability, light attenuation behavior and repetition stability thereof are deteriorated due to exposure of the photoreceptor to external light during coating/after-processing or at the replacement or maintenance, a measure for blocking light as much as possible is sometimes taken from a hardware side, but this is disadvantageous in view of cost and it is most preferred that the photoreceptor itself is basically not subject to light-induced fatigue. On the other hand, most of recent electrophotographic photoreceptors are an organic photoreceptor and cannot be said to have an adequate measure for light-induced fatigue characteristics, and the risk of allowing a slightly produced potential variation to emerge as an image defect is high.
As for the measure against light-induced fatigue of the photoreceptor, various techniques have been heretofore proposed. Patent Document 1 discloses a method of adding a dissimilar charge transport material having a small ionization potential, where the potential variation immediately after exposure to light is decreased but when use is further repeated, the dissimilar charge transport material having a small ionization potential works as a trap for a charge carrier, giving rise to a problem that residual potential is accumulated. Patent Document 2 discloses a technique of improving the repetition stability by the addition of an acceptor compound. In this case, the potential in repeated use is stabilized, but the potential variation (potential reduction) immediately after light irradiation is disadvantageously large. Patent Document 3 discloses a technique of incorporating a compound such as pigment having an absorption peak at 450 to 620 nm into a charge transport layer. However, in the case of adding a known pigment or dye, there exists a problem in terms of dispersion stability in the coating solution or a problem that the side effect such as performance variation and image defect is significant. Patent Document 4 discloses a technique of adding an azo compound having a charge transport function and having a strong absorption in the visible region. In this case, use of the compound is sometimes prohibited depending on the compatibility with various charge transport materials, and the technique cannot be said necessarily to be a versatile technique. Patent Document 5 discloses a technique of adding an ultrafine particle, where blue light is prevented from reaching the inside of the photosensitive layer by a flip-flop effect and the characteristics are less likely to be changed or deteriorated. In this case, the dispersion stability of fine particle in a coating solution is again a problem, and not only the mass production is limited but also the light-shielding effect on the wavelength side longer than blue light cannot be satisfied.
Incidentally, it has been proposed to use a specific fluorene compound and a specific diamine compound in combination for a charge transport material which is one of constituent materials of the photoreceptor, and thereby realize inexpensive production of a photoreceptor excellent in the balance of performances such as high sensitivity, stable potential in repeated use, good crack resistance and less occurrence of transfer memory (Patent Document 6). However, these two kinds of charge transport materials are susceptible to the above-described light-induced fatigue, and there is a problem that in order to prevent an image trouble, for example, control of the light quality in the production line, filtering of short-wavelength light, winding of a light-shielding film around the photoreceptor or installation of a light-shielding mechanism in the cartridge is required, leading to lost productivity and rising cost.